Patent Document 1 describes a resin composition containing polyvinyl alcohol and filler particles as a resin composition for a surface treatment of a separator substrate for a non-aqueous electrolyte secondary battery. However, a separator, which is obtained by using the resin composition as a resin composition for a surface treatment of a separator substrate for a non-aqueous electrolyte secondary battery, does not necessarily have adequately satisfactory heat resistance. It is an object of the present invention to provide a separator having excellent heat resistance.